UV-VIS-IR-sensitive photopolymer compositions have been used extensively in many applications in the area of photolithography, graphic art, stereolithography, and printing and publishing. All of these applications require materials that can be polymerized imagewise; that is, the polymerization reaction is spatially confined to the region irradiated by the photons to retain the input image with good fidelity and spatial resolution. Because of the short penetration depth (in absorbing media) and scattering problems of optical photons, the use of relatively thin and transparent photopolymer films is usually required for these applications. Opaque medium is very problematic for this technology. For example, conventional photopolymer technology is not suitable for the patterning of ceramic materials.
These issues may be resolved by the development of useful x-ray sensitive photopolymers. X-ray has deeper penetration depth and, in the case of lithography, can achieve better spatial resolution than optical based technique. Unfortunately, no material has been developed that can be polymerized imagewise by a relatively low intensity x-ray beam within a reasonably short duration, as best known to the inventor. Organics have very low x-ray absorption coefficiency and polymerization reaction if can be initiated by x-rays, tends to be inefficient and requires the use of either very long exposure time or high power x-ray source, such as synchrotron radiation. In addition, for applications requiring spatial resolution, the polymerization reaction has to be spatially confined to the irradiated region.
There are many potential applications for x-ray sensitive photopolymers if these materials are available. The microfabrication of ceramics and metals (for example, barrier rib fabrication for plasma flat panel display; S. W. Depp and W. E. Howard, Sci. Amer. 260,40 March 1993), stereolithography (3D-solid object modeling) (D. C. Neckers, Chemtech, October issue, p. 615, 1990) and photoresist for x-ray or e-beam or ion beam lithography (C. Grant Wilson in Introduction to Microlithography, eds. L. F. Thompson, C. G. Wilson, and M. J. Bowden, 1994, American Chemical Society, Chapter 3, p.139) are just a few examples. Other applications include x-ray contact microscopy (Applied Physics Letters, 72, 258 (1998) by A. C. Cefalas, P. Argitis, Z. Kollia, E. Sarantopoulou, T. W. Ford, A. D. Stead, A. Marranca, C. N. Danson, J. Knott, and D. Neely) and the fabrication of photonic crystals with photonic band gap properties (Science, 281, 802 (1998), by J. E. G. J. Wijnhoven and W. L. Vos).
The basic principles of initiating chemical reactions with ionization radiation can be found in “Radiation Chemistry” by A. J. Swallow, Wiley, 1973; and “Pulse Radiolysis” by M. S. Matheson and L. M. Dorfman, MIT Press, 1969. Here ionization radiation is defined to include x-rays, γ-rays, neutrons, charged particles (ion beam), and electron beam. The irradiation of matters with ionization radiation can generate excited states, free radicals, cations, and anions. Under the proper conditions, these reactive species can initiate chemical reactions such as polymerization, cross-linking, and bond breaking.
Several patents disclose the use of gamma ray radiation for industrial applications. U.S. Pat. No. 3,950,238 (Apr. 13, 1976, R. J. Eldred) discloses an acrylonitrile-butadiene elastomer composition that can be cured by an electron beam. U.S. Pat. No. 4,004,997 (Jan. 25, 1977, Tsukamoto, Matsumura, Sano) discloses a process of curing a resin filled with powdery ferromagnetic substance using radioactive rays. U.S. Pat. No. 4,303,696 (Dec. 1, 1981, Brack) describes a method of curing a liquid prepolymer composition to form a waxy, release coating on a solid surface. U.S. Pat. No. 4,319,942 (Mar. 16, 1982, W. Brenner) discloses electron beam curing of adhesive compositions containing elastomers for building flocked composite structures. U.S. Pat. No. 4,353,961 (Oct. 12, 1982, Gotcher, Germeraad) discloses radiation cross-linking of fluorocarbon polymer to improve the mechanical strength. U.S. Pat. No. 4,547,204 (Oct. 15, 1985, Caul) discloses resin compositions, such as acrylated epoxy and phenolic resin, which can be cured by electron beam for coated abrasive application. U.S. Pat. No. 5,098,982 (Mar. 24, 1992, Long) discloses that the hardness of thermoplastic polyurethanes can be improved upon irradiation by electron beam. U.S. Pat. No. 5,037,667 (Aug. 6, 1991, Dubrow, Dittmer) discloses that certain organopolysiloxanes can be grafted to polymeric supports by the irradiation with electron beams. U.S. Pat. No. 5,332,769 (Jul. 26, 1994, Kakimoto, Eguchi, Kobayashi, Nishimoto, Iseki, Maruyama) discloses electron beam curing of adhesives for adhesion between a polyester film and a metal plate. J. Polym. Sci., XLIV, 117-127(1960) by B. Baysal, G. Adler, D. Ballantine, and P. Colombo discloses solid state polymerization of acrylamide initiated by gamma ray radiation to produce polyacrylamide. There was not enough solubility differentiation between the starting material and the product to allow spatially defined image formation. U.S. Pat. No. 4,115,339 (Sep. 19, 1978, by A. J. Restaino) discloses gamma ray initiated polymerization of nitrogen-containing vinyl monomers to form aqueous gels of water-soluble polymers. Both the starting material and the product are water-soluble so spatially defined image formation cannot be achieved.
All of the above patents and papers disclose methods or compositions used mostly for coating and adhesion applications under gamma ray radiation. The efficiency was generally low and the required spatial resolution for imaging applications was not demonstrated.
Recently, synchrotron radiation has been used to cross link polymethyl-methacrylate for the precision machining of solid parts (Johnson, Milne, Siddons, Guckel, Klein, Synchrotron Radiation News, 9, 10 (1996)). The intensity of synchrotron radiation is roughly 1 million times higher than the common laboratory x-ray machines such as the ones used in this work. The extremely high intensity of the synchrotron radiation means the polymerization/cross-linking reactions can be initiated from almost any compositions (that is, it is non-discriminate). The high intensity also presents damage and heating problems with the mask that needs to be used for imaging applications. Because of the cost of the synchrotron machine and the limited availability, the use of synchrotron radiation is not practical. The compositions disclosed in this invention allow the use of commonly available, low intensity x-ray machine (or e-beam and gamma ray) to achieve spatially defined polymerization/cross-linking reactions.
U.S. Pat. No. 5,556,716 (Herron and Wang) discloses x-ray sensitive photoconductive compositions for digital radiography applications. The compositions comprise of hybrids of organic polymers and inorganic nanoparticles. Unlike the materials disclosed in the present invention, x-ray generated electrons and holes in these photoconductors do not induce any chemical reactions; they are separated and transported out of the film under high fields.